Additional fixes and improvements to JavaClassWrapper

- Fix crashing bug when invoking class constructor with parameters
- Add support for accessing class constants
- Add support for Godot Callable arguments. A Godot Callable can be wrapped by a Java Runnable to allow Java logic to run arbitrary Godot lambdas
- Automatically convert java.lang.CharSequence to Godot String as needed
- Code cleanup

platform/android/api/api.cpp

@@ -73,6 +73,9 @@ void JavaClassWrapper::_bind_methods() {
 }
 
 #if !defined(ANDROID_ENABLED)
+bool JavaClass::_get(const StringName &p_name, Variant &r_ret) const {
+	return false;
+}
 
 Variant JavaClass::callp(const StringName &, const Variant **, int, Callable::CallError &) {
 	return Variant();